Digitally controlled DC-DC converters have shown more flexibility over their analog counterparts with the introduction of intelligent control techniques. The intelligent control techniques include, for example, the use of an auto-tuning system that can tolerate passive component variations as described in “Limit-cycle oscillations based auto-tuning system for digitally controlled DC-DC power supplies”, by Z. Zhao, A. Prodić, IEEE Trans. Power Electronics, vol. 24, no. 6, pp. 2211-2222, November 2007, the use of a segmented output stage that can dynamically adjust the size of the output transistors according to load conditions to maintain high power conversion efficiency as described in “A digitally controlled DC-DC converter module with a segmented output stage for optimized efficiency”, by O. Trescases, W. T. Ng, H. Nishio, M. Edo and T. Kawashima, Proc. Int. Symp. Power Semiconductor Devices and ICs, June 2006, pp. 373-376, and the one-step dead-time correction that can optimize the turn-on and turn-off dead-time for power transistors on-the-fly as described in “One-step digital dead-time correction for DC-DC converters”, by A. Zhao, A. A. Fomani and W. T. Ng, Proc. Applied Power Electronics Conf., February 2010, pp. 132-137. In addition, digitally controlled DC-DC converters have the ability to switch seamlessly between linear and nonlinear operation modes and achieve near-optimal load transient performance as described in “Minimum deviation digital controller IC for single and two phase DC-DC switch-mode power supplies”, by A. Radic, Z. Lukic, A. Prodic and R. de Nie, Proc. Applied Power Electronics Conf., February 2010, pp. 1-6.
As power supply requirements for microprocessors become more stringent, however, the design of power converters has become more challenging. Point-of-load (POL) DC-DC converters driving modern microprocessors need to provide low output voltage, high output current and good dynamic performance during load transients, while at the same time maintaining high efficiency. Smaller LC filters and higher switching frequency or multiphase/interleaved structure have been proposed to improve the converter's transient performance. See, for example, “Critical Inductance in Voltage Regulator Modules”, P. L. Wong, F. C. Lee, Px Xu and K. Yao, IEEE Transaction on Power Electronics, Vol. 17, No. 4, July 2002, pp. 485-492. However, these solutions usually suffer from efficiency degradation. In order to address this problem, an additional power output stage with much smaller filter inductance has been added to the main converter to reduce the output voltage overshoot without deteriorating the steady-state efficiency. See, for example, “A fast transient recovery module for DC-DC converters”, by P. J. Liu, H. J. Chiu, Y. K. Lo, and Y.-J. E. Chen, IEEE Trans. Industrial Electronics, vol. 56, no. 7, pp. 2522-2529, July 2009, the content of which is incorporated herein by reference. However, depending on the implementation, the use of auxiliary stages can take up valuable space.
A digitally controlled transient suppression method involving an auxiliary output stage connected in parallel with the main output stage has been proposed in “A Digitally Controlled Transient Suppression Method for Integrated DC-DC Converters”, K. NG, J. Wang, and W. T. Ng, IEEE International Conference on Electron Devices and Solid-State Circuits (EDSSC), December 2008, the content of which is incorporated herein by reference. In the digitally controlled transient suppression method, a capacitor charge balance principle is applied to bring the output voltage back to within a tolerable window in a single switching cycle, which in turn results in a very short recovery time. The auxiliary stage is used to assist the sinking or sourcing of the load current, which helps to restore the output voltage quickly when load transient occurs. It is desirable that that the auxiliary power transistors be much smaller than those in the main output stage. As a result, the total area required for dual output stages does not impose a significant overhead when compared to converters with a conventional single phase output stage, thereby making the auxiliary stage a method viable for integration.
In view of the above, it would therefore be desirable to provide a fully integrated DC-DC converter that achieves a fast load transient recovery, while at the same time achieving well balanced power conversion efficiency and dynamic performance without a significant area penalty.